Sorry for the late reply on the mail u feedback to me. Can u tell me which configuration file I should check. Is it httpd.conf (in Apache directory) or ? I check my ServerName is <IP_address>.
I am able to access until Oracle HTTP Server Components screen.

But once I choose the mod_plsql -> Home (Gateway Configuration Menu) - error displayed as below
Client Logon Failed with errorcode:28150

Error TimeStamp:Tue, 9 Jan 2001 05:13:40 GMT
Database Log In Failed
TNS is unable to connect to destination. Invalid TNS address supplied or destination is not listening. This error can also occur because of underlying network transport problems.

Verify that the TNS name in the connectstring entry of the DAD for this URL is valid and the database listener is running.

Is this problem related to the tnsnames.ora file ?

ORA-12545: Connect failed because target host or object does not exist - This error message still exist in my sqlplus.

Another question is, do I really need the domain server for testing purposes ? Currently my DNS is disabled.